How to Combine two Normal Maps?
I adapted the UDN Blending method from this article, for cycles nodes.

The simple formula is:
"Normal map 1" (nm1) the large distortion, "Normal map 2" (nm2) the small details.
nm1.x + nm2.x = X, nm1.y + nm2.y = Y, nm1.z = Z




Split the normal maps in the their three separate channels with a Separate XYZ node.
Add the X channel of both normal maps together with a Math node set to Add
Add the Y channel of both normal maps together with a Math node set to Add
Make the new vector with a Combine XYZ node. Plug the component X into the X input of the Combine XYZ node, and the component Y into the Y. Take the Z from the first normal map.
Add a Vector Math node set to Normalize. Take the output from the Combine XYZ node and plug it into the first slot of the Normalize node.
Add a Normal Map node. Take the output from the Normalize node and plug it into the color slot of the Normal Map node.




Another option is to use Multiply instead of Add for combining the X and Y channels. The bonus with this is, I have found no need for the Normalize node. The resulting normal map will look different from the method above.